This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Human digestive carboxypeptidases CPA1, CPA2 and CPB1 are secreted by the pancreas as inactive proenzymes containing a 94-96 amino-acid long propeptide. Activation of procarboxypeptidases is initiated by proteolytic cleavage at the C-terminal end of the propeptide by trypsin. In this study, we have demonstrated that subsequent cleavage of the propeptide by chymotrypsin C (CTRC) induces a nearly 10-fold increase in the activity of trypsin-activated CPA1 and CPA2, whereas CPB1 activity is unaffected. Other human pancreatic proteases such as chymotrypsin B1, chymotrypsin B2, chymotrypsin-like enzyme-1, elastase 2A, elastase 3A or elastase 3B were found to be inactive or markedly less effective at promoting procarboxypeptidase activation. On the basis of these observations we propose that CTRC is a physiological co-activator of proCPA1 and proCPA2. Furthermore, the results confirm and extend the notion that CTRC is a key regulator of digestive zymogen activation.